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McKenney writes: > On Wed, Oct 09, 2024 at 12:05:47PM -0700, Ankur Arora wrote: >> >> >> Ankur Arora writes: >> >> > With PREEMPT_RCU=n, cond_resched() provides urgently needed quiescent >> > states for read-side critical sections via rcu_all_qs(). >> > One reason why this was needed, was lacking preempt-count, the tick >> > handler has no way of knowing whether it is executing in a read-side >> > critical section or not. >> > >> > With PREEMPT_LAZY=y, there can be configurations with PREEMPT_COUNT=y, >> > PREEMPT_RCU=n, where cond_resched() is a stub that does not provide >> > quiescent states via rcu_all_qs(). >> > >> > So, use the availability of preempt_count() to report quiescent states >> > in rcu_flavor_sched_clock_irq(). >> >> A note about the inverse of this case, where we might have long running >> loops which only temporarily enable preemption and thus would be >> unlikely to align themselves with the tick: in prior discussions [1] >> Paul had pointed the need for providing for forcing a context switch >> in such a scenario. >> >> I had a patch which did that, but I think it is unnecessary since this >> clause in rcu_sched_clock_irq() should already handle it. >> >> void rcu_sched_clock_irq(int user) { >> ... >> /* The load-acquire pairs with the store-release setting to true. */ >> if (smp_load_acquire(this_cpu_ptr(&rcu_data.rcu_urgent_qs))) { >> /* Idle and userspace execution already are quiescent states. */ >> if (!rcu_is_cpu_rrupt_from_idle() && !user) { >> set_tsk_need_resched(current); >> set_preempt_need_resched(); >> } >> __this_cpu_write(rcu_data.rcu_urgent_qs, false); >> } >> >> Paul? > > As long as the tick is actually enabled. > > But looking deeper, there is code in force_qs_rnp() and > rcu_watching_snap_recheck() to force the tick on CPUs that don't > response to the grace period soon enough via the -1 return from the > rcu_watching_snap_recheck() function and via resched_cpu(). Ah. I had missed that path. Thanks. -- ankur